Electric resistance element and process of manufacturing same



May 2o`, 1941. c. @no 2,242,344

ELECTRIC RESISTANCE ELEMENT AND PROCESS OF MANUFACTURING SAME Filed Dec. 17, 1938 2 Sheets-Sheet 1 C. CITO May 20, 1941.

ELECTRIC RESISTANCE ELEMENT ANDVPROCESS OF MANUFACTURING SAME Filed' Dec. 17, 1938 2 Sheets-Sheet 2 Patented May 2o, 1941 .ELECTRIC RESISTANCE ELEMENT AND PROCESS F MANUFACTURING SAME Camillo Cito, Uccle-Brussels, Belgium, assignor to Manufacture Generale Metallurgique, Socite Anonyme, Brussels, Belgium, a Belgian company' Application December 17, 1938, Serial No. 246,462

' In Belgium December 20, 193'!` 12 Claims.

'Ihls invention relates to electric resistance elements of the type in which a suitably insulated high resistance conductor surrounds a supporting center piece and is in turn surrounded by a tubular metal sheath.

An object of my invention' is to providel a tighter tit of the sheath about'the resistance and a better 'thermal contact between the ilns and the metal forming the sheath, in order to promote the transmission of heat. To this end I form the tubular sheath of the electric resistance element by casting about a core threaded axially through the holes in the ns a metal with a high contraction which, on cooling, shrinks and causes the sheath strongly to grip the core, while providing a flrm anchorage for the fins. The core comprising the insulated vconductor and its supporting center piece, :will resist the contraction .of the sheath so that an eiiicient compression is A further object of my invention is to pro-r vide a resistance element of this.:.type that will in use be free from stresses due to excessive internal expansion, and to this end I wind the high resistance conductoron a tubular center piece which has staggered cross slits, in order to oppose its tendency to expand lengthwiseunder the action of heat, when the sheath is cast.

It is also an object of my invention to facilitate the casting of the sheath by forming the fins with notches or serrations along the edges of their holes, so as to provide around `the core a number of passages allowing of the molten metal flowing along the core past the fins during the casting operation.

Still another object of my invention is to make it possible to dispense with casting molds and to this end, according to a preferred method of carryingv out my inventlon, the metal of the sheath is cast around the core in an annulated shell, or a shell made of ring-shaped sections formed by theilns themselves in conjunction with intermediate rings, which shell is permanently left around and thus forms part of the element. I thus am enabled to cast elements of any desired length without being limited by the 'fixed dimensions of a length without being limi'ted by the fixed dimensions of a mold, since the length of the shell can be increased or decreased by increasing or decreasing the) number of fins and intermediate rings.

"Preferably the 'intermediate rings are integral with the fins, said'rings then being in the form of annular projections or bosses extending around the hole adapted to receive the core and the cast metal. These projections or bosses are preferably so shaped that the boss on one iin closely iits into the hole in the following fin, to form a continuous shell.

For casting the metal that is to forml the sheath, I place the shell between two movable heads which are adapted to close its ends and make it fluid-tight, said heads axially clamping the shell, centering the core and permitting of introducing the metal to be cast. 'The heads are preferably clamped elastically in order to compensate for axial expansion and contraction of the shell during casting.

In the accompanying drawings illustrating by way of example electric resistance elements constructed in accordance with my present invention:

Fig. 1 is a sectional elevation of an element showing one end thereof provided with connection means,

' Fig. 1a is a sectional elevation illustrating the opposite end, Fig. 2 is a cross section on line II-II of Flg. 1. Fig. 3 is a front view of a fin with an annular projection or boss.

Fig. 4 is an axial sectional view of a plurality of aligned fins such as shown in Fig. 3, the ns being in mutual engagement.

Fig. 5 is a sectional elevation showing both ends of an electric resistance element with its metal sheath cast in a section shell and mold heads.

Figs. 6, '7, 8 and 9 are respectively cross-sectional views similar to Fig. 4, illustrating various embodiments of the invention.

The electric resistance element shown in Figs. 1, 1a and 2 comprises a tubular supporting body or center piece I with transverse slits 2 formed therein in staggered relation, an electric resisttance member 3 wound on said center piece between two insulating layers 4, 5 of wound mica sheets, and a tubular sheath 6 of cast metal carform through the succession of ns free passages which facilitate the flow of the molten metal along the core, from one end of the mold to the other end, to form the sheath 6.

'I'he contraction of the metal forming the sheath exerts an inward pressure on the core and presses the resistance winding 3 and the insulating layers 4, 5 lagainst the tubular center piece I, while the slits 2 in said piece permit of the tubular sections therebetween expanding and contracting under the influence of temperature variations without appreciably aiecting the total length of the tubular piece. l

Rings I0 and II forming terminal connections to the electrical circuits are cast like the sheath 3. Both rings are secured in position by the contraction of the metal, the ring I0 being clamped on the resistance 3 which is bared at that place, and the ring II being clamped on the tubular center piece I which is also bared to receive it (Fig. 1). I2 is an insulating sleeve.

. At the other end of the element the sheath E completely surrounds an insulating plug I3 at I the end of the tube I, which end is electrically connected at I4 to the end of the resistance 3, the insulating layer 4 being cut away, so that the tubular piece I is used as a return conductor. When the electric resistance element is branched on a circuit, the electric current flows through terminal I0 and the resistance member 3 and returns through the tubular piece I to terminal An excellent heattransmission is ensured from resistance member 3 to sheath 6 and from the latter to the fins 1, owing to the tight grip ,of the sheath about the resistance which excludes any air pocket capable of hindering thermal conduction, and owing to'the intimate contact of the fins with the metal of the sheath in which they are partly embedded.

In the examples shown in Figs. 3, 4 and 5, each iin 2I has around its hole 22 an annular boss or projection 23 which may be formed by stamping. This cylindrical boss 23 has a portion 24 of smaller diameter which'forms a step and it terminates in an inwardly lprojecting flange 25 in -which are cut four notches 26. The stepped portion 24 is uted at 21 and its diameter is such that the,step 24 of one iin fits exactly into the boss 23 of the next fin (Fig. 4), while the shoulder 23 limits the depth of engagement and ensures a uniform spacing of the fins.

, It will be understood that by the mutual engagement of a suitable number of fins provided with bosses forming intermediate rings. an annular shell of any desired length will be obtained.

The shell' being built up, I place therein the core comprising for example a tubular center piece 29 and a resistance member 30 wound between two insulating layers 3I, 32. The core is supported at both ends by mold heads 33, 34 which also support the annular shell in axial relation thereto and clamp same, the clamping action being exerted by springs 35 coiled around bolts 36 and adapted to press towards each other a cheek 31 carrying the head- 33 and a cheek 38 carrying the head 34.

Each head 33, 34 may be made of two or more parts and the head 33 has a hole 39 through which the metal 40 which is to form the tubular sheath is cast aroundthe core in the annulated shell. The injected metal flows easily along the core through the passages afforded by the notches 25 in the flanges 25, until it fills the shell and form vents along the joints between successive fins, so that the formation of blisters in the =cast metal will be avoided. Inthe cast metal are embedded the flanges 25 of the rings or bosses which thus provide an anchorage for the iins 2I in the sheath 40.

The mold heads 33, 34 are opened in order to disengage the ange 4I and the blind end 42 of the sheath 40, and then the finished resistance element is removed from between the cheeks 31, 38.

Instead of fins having stepped cylindrical bosses as above described, I may use fins associated with bosses or rings of other shapes, which are integral with or secured to the fins and adapted to fit into or against each other, or I may also use fins associated with separate insertion rings.

In Fig. 6, the fins 2Ia have conical bosses 23a the mutual engagement of which is limited by abutment lugs 43 bent inwardly from the collars 25a, said lugs also affording an additional anchorage for the fins in the cast metal.

A very reduced engagement' may be suicient to keep the iins in alignment as shown in Fig. '7, in which the. fins 2 Ib have bosses 2317 in the form of ring-shaped beads terminating in collar 25h.

In the embodiment shown in Fig. 8, the fins 2Ic have ilaring bosses 23e and collars 25c which do not fit into each but are merely in mutual contact. Their correct alignment may be 0btained by threading them on a mandrel before they are clamped between the mold heads in order to form an annulatedshell.

Likewise, fins ZId and separate spacing rings 44 may be aligned and clamped together, as shown in Fig. 9, wherein the collar 25d are formed on the ns themselves.

In some cases the collars or other anchoring means provided on the fins or on their bosses or spacing rings may be omitted, the resiliency of the metal of the fins, bosses or rings being relied upon to hold them tight on the cast sheath, despite the shrinkage of the metal forming the sheet.

Electric resistance elements, constructed according to this invention are suitable for use as heating resistances, starting resistances, brakresistances or other uses. The invention of course' is not limited to the constructional details herein shown, and modication may be made without departing from its scope as defined by the appended claims.

I claim:

1. In a process of making an electric resistance element, forming a core by surrounding an elongated center piece with -a resistance men.- ben between two layers of insulating material, aligning a. plurality of iins having each a central hole of larger diameter than said core, positioning said corel in the aligned holes of said fins, and casting metal around said core and in contact with the inner edges only of said fins, thus forming a sheath shrunk on said core and adapted to hold said fins.

2. In a process of making an electric resistance element, forming a core by surrounding a cylindrical center piece with a resistance member between two layers of insulating material, aligning a plurality of fins having each a central hole of larger diameter than saidV core and notches in the inner-edges 'of said fins, centering said core in the aligned holes of said fins, providing a tubular space around said core, and casting metal from one end of'said space along said core and in contact with the notched edges oi' said ns without covering the outer edges of the ilns so as to form a sheath shrunk on said core and adapted to hold said ilns.

3. In a process of making an electric resistance element, forming a core by surrounding a metal center piece with a resistance member between two layers of insulating material, aligning a plurality of ilns having each a central hole of larger diameter than said core, positioning said core in the aligned holes oisaid ilns, providing a tubular space around said core, electrically connecting said member and said center piece at one end, casting a tubular metal sheath around said core and in contact with the inner edges only oi'said ns, and casting terminal blocks about the bared end of said resistance member and the bared end lot said center piece respectively.

4. In a process of making an electric resistance element, forming a cylindrical core by surrounding a center piece with a resistance member between two layers oi.' insulating material, forming an annular shell by aligning a plurality oi' ns and rings in alternating relation, said ns and rings enclosing a space oi' larger diameter than said core, positioning said core in said space, and

r casting metal in said space without covering the outer edges of the ,ilns so as to form-around said core a sheath rigid with said fins and rings.

5. In a process of making an electric resistance element, forming a cylindrical core by surrounding a center piece with a resistance member betwene two layers of insulating material, forming an annular shell by aligning a plurality oi' ilns and rings in alternating relation, said ilns and rings enclomg a space of larger diameter than said cor'e, positioning said core in' said space, axially clamping said ns and rings, closing said space at both ends, and casting metal into said space without covering the outer edges of the ilnsmastotormaroundsaidcoreasheathrigid wisth said nns and rings.

nation ot an elongated center piece of conduct-v ing material, an inner insulating layer around said center piece, one end of said center piece being bare, a resistance coil surrounding said inner layer, an outer insulating layer around said coil, the end oi.' said coil adjacent the bare end of said center piece also being bare, the respective opposite ends of said center piece, and said coil being electrically connected with each other, terminal blocks cast about the bare end of said center piece and the bare end oi' said coil respectively, a metal sheath cast around said outer insulating layer, and ilns partly embedded in said sheath.

l8. In an electric resistance element, the combination ol a tubular center piece, said center piece being provided with circumferentially extending slits axially spaced therealong, an inner insulating layer around said center piece, a resistance member surrounding said inner layer, an outer insulating layer surrounding said member. a metal sheath cast around said outer insulating layer, and annular ilns having their inner edges` only embedded in said sheath.

9. In an electric resistance element, the combination o! a core. a shell and a tubular metal sheath cast between said core and said shell, said core comprising acenter piece, two insulating layers and a resistance member between` said layers, and said shell comprising a succession of tins and rings integral each with one of said ns.

10. In an electric resistance element according to claim 9,- the respective rings forming annular bosses on the respective fins, the boss on one tin being engaged in a hole in the next iln.

1l. In an electric resistance element according to claim 9. said rings having each inwardly extending projections embedded in the metal forming Said Shell. 

